This invention relates to an aluminum base bearing alloy which contains at least one soft, lubricating element such as Pb, Sn and/or Sb, Si as a hard element and at least one reinforcing element such as Cu and/or Cr and has improved fatigue resistance, and to a method of producing the bearing alloy.
Some kinds of copper base alloys such as Cu-Pb base alloys and Sn-Sb-Cu base alloys (Babbitt metal) have long been used as the bearing alloys for plain bearings in various machines. In recent years, lightweight aluminum base bearing alloys have been attracting increasing attention particularly for use in internal combustion engines in which bearing alloys are required to be high in heat resistance, wear resistance, corrosion resistance and fatigue resistance. Particularly, Al-Sn base and Al-Sn-Pb base bearing alloys are fairly better than other aluminum base alloys in the aforementioned endurance characteristics, so that proposals and practical applications of these bearing alloys are rapidly increasing. For example, Japanese patent application primary publication No. 58-171545 (1983) shows an Al-Pb-Sn base bearing alloy which contains Si as a hard component and at least one of Ni, Mn, Cr, V, Mg, Ti, Zn, Co and Zr as a reinforcing component and which is produced by compacting a powder mixture of the constituent elements and/or their alloys with aluminum or lead and extruding the compacted preform after heat treatment.
With the advancement and sophistication of internal combustion engines and particularly of automotive engines, severer conditions are enforced on the bearings in the engines. For example, widths of bearings are reduced as the gross size of the engine is reduced, and loads on bearings are increased as the engine output is increased. Accordingly still there is a strong demand for development of superior aluminum base bearing alloys. Especially it is keenly demanded that aluminum base bearing alloys should be improved in fatigue resistance since conventional aluminum base bearing alloys are liable to crack or locally peel off the backing metal within a period not long enough from a practical point of view.
To meet the aforementioned demand, in Japanese patent application primary publication No. 61-12844 published Jan. 21, 1986 we have disclosed an aluminum base bearing alloy which is excellent in both lubricating capability and fatigue resistance. This bearing alloy contains at least one of Pb, Sn, In, Sb and Bi as a lubricating component, Si as a hard component and at least one of Cu, Cr, Mg, Mn, Ni and Zn as a reinforcing component. The lubricating component is uniformly and finely dispersed in the aluminum matrix and amounts to 0.006-0.040 by sectional area ratio to the aluminum matrix, and the grains of this component are not larger than 8 .mu.m. Si dispersed in the aluminum matrix amounts to 0.003-0.060 by sectional area ratio to the aluminum matrix and is not larger than 12 .mu.m in grain size. The reinforcing component amounts to 0.2-5.0 wt%. The bearing alloy is required to be not lower than 15 kgf/mm.sup.2 in tensile strength at normal temperature and not less than 13.5% in elongation at normal temperature. This bearing alloy is produced by compacting a mixture of raw material alloy powders into a billet and extruding the billet at a suitable temperature at an extrusion ratio not lower than 10.
The aluminum base bearing alloy according to JP 61-12844 exhibits excellent bearing characteristics so long as the lubricating oil is almost free from hard foreign matter. However, this bearing alloy is not very high in the ability to embed foreign matter and accordingly offers a problem that the bearing capability lowers when a considerable amount of foreign matter enters the lubricating oil. There is another problem. Sometimes and particularly when the mating material is cast iron, the aluminum base bearing alloy is scratched by the tiny burrs existing on the machined surface of the mating material mainly around the particles of free carbon.
In aluminum base bearing alloys containing Si as a hard element it is desirable that the grain size of Si is not excessively small from the viewpoint of enhancing wear resistance of the bearing alloy. In the case of producing a bearing alloy of this type by extrusion of a compacted alloy powder mixture, usually the extruded alloy needs to be subjected to a heat treatment to allow very fine grains of Si contained in the starting powder to a suitable level such as about 10 .mu.m. However, this treatment is not very easy when the bearing alloy contains relatively large amounts of lubricating elements such as Pb and Sn because the heat treatment is liable to cause exudation of the low melting point lubricating elements such as Pb and Sn onto the alloy surface, which is known as a sweating phenomenon.